1. Field of the Invention
The present invention relates to an amplifier circuit and a communication device.
2. Description of the Related Art
In an architecture of a radio receiver, since an input signal is low, an amplifier for amplifying the signal is required in the first stage. Generally, the noise is generated from the electric circuit. If the noise level generated from the circuit is large, the signal to noise ratio becomes small. Therefore, the amplifier with a small noise generated from the circuit is requested. The larger a gain of the amplifier becomes, the more effective it is for lowering the equivalent input noise. As an amplifier structured to have a large gain, a common-source amplifier is generally used.
Input impedance of the common-source amplifier is high. In the architecture of the radio receiver, it is required not only to lower the noise but also to match the input impedance with an impedance of an antenna and transmit a signal being incident from the antenna to the amplifier at the maximum (impedance matching).
Further, the low-noise amplifier generally requires a variable gain function since there is a need to stabilize the gain and send the signal to a circuit (mixer) at a later stage.
FIG. 8 is a block diagram showing a structural example of a low-noise amplifier, and FIG. 9 is a circuit diagram showing a structural example of a low-noise amplifier in which a common-source amplifier is used. A voltage controlled current circuit 801 being a common-source amplifier composed of cascode-connected field effect transistors 901 and 902, converts an input voltage Vin into a current and outputs it. An output load 802 converts the current converted in the voltage controlled current circuit 801 into an output voltage Vout. A buffer 803 has a field effect transistor 903 and a current source 904, and feedbacks the output voltage Vout to a terminal of the input voltage Vin via a feedback circuit (element) 804.
An input impedance Zin of the low-noise amplifier can be represented by the following formula. Here, “Zfb” is an impedance of the feedback circuit 804, “gm” is a transconductance of the field effect transistor 901, and “Zout” is an impedance of the output load 802.Zin=Zfb/{1+(gm×Zout)}  (1)
Here, when a formula of gm×Zout>>1 is satisfied, the formula (1) can be represented by a formula (2).Zin=Zfb/(gm×Zout)   (2)
The input impedance Zin is determined by the transconductance gm, the impedance Zout of the output load, and the impedance Zfb of the feedback circuit 804.
Japanese Patent Application Laid-open No. 2004-159195 discloses an amplifier circuit provided with a common-source transistor which amplifies a signal being input into a gate terminal and outputs it from a drain terminal, a load resistance connected to the drain terminal of the common-source transistor, an inductor connected in series with the load resistance, and a feedback resistance connected between the drain terminal and the gate terminal of the common-source transistor.
Further, Japanese Patent Application Laid-open No. Hei 9-246880 discloses an amplifier circuit having a common-source amplifier circuit composed of a first transistor which performs a signal amplification and a first active load formed of an FET, a source follower circuit composed of a second transistor which inputs a drain output signal of the first transistor and a first current source, a feedback resistance connecting an output terminal of the source follower circuit and an input terminal of the common-source amplifier circuit, a capacitance connecting the input terminal of the common-source amplifier circuit and a gate terminal of the FET composing the active load, and an impedance element connecting the gate terminal of the FET composing the active load and a power supply.
The variable gain function is provided in circuits shown in FIG. 8 and FIG. 9, so that when the transconductance gm of the transistor 901 is changed or when the output load 802 has a frequency dependency, and the like, a denominator of the aforementioned formula (2) is changed, resulting that the input impedance Zin cannot be kept constant in these circuits. Accordingly, a mismatch between the input impedance Zin and the impedance of the antenna is generated and, consequently, it becomes impossible to transmit the signal sufficiently.